Photolithography, defined as a process for effecting the photographic transfer of a pattern to a surface for etching or implanting, is employed in the fabrication of myriad types of semiconductor devices, including integrated circuit (IC) devices. In general, photolithography involves the performance of a sequence of process steps, including coating a semiconductor wafer with a resist layer, exposing the coated wafer to a patterned light source, developing the resist layer, processing the semiconductor wafer through the developed resist layer, and removing the resist layer. An optical photolithography stepper apparatus, or "stepper," such as those available from ASM Lithography, Inc., located in Eindhoven, Netherlands, is typically used to expose the resist layer. An image of each layer of an IC die is formed on a small, rectangular piece of glass referred to as a reticle, or "mask." The mask is placed on the stepper and a reduced image thereof is projected onto a portion of the resist layer covering the semiconductor wafer.
Where numerous ICs are to be fabricated from a single wafer, a mask used in the fabrication of any one IC is also used in the fabrication of the other ICs from the wafer. This is accomplished by using a stepper to index, or "step," the wafer under an optical system including the mask and a projection lens, in the x or y direction by a predetermined pitch. At each step, the photoresist is exposed by the optical system, typically with ultraviolet light, to form an image of the mask in the layer of photoresist. Once the entire wafer has been stepped, the wafer is then removed from the stepper and the image developed. At that point, the wafer is etched to remove portions of the underlying film or implant to prepare the wafer for the next stage of material deposition or other types of etching processes. At a later stage in the fabrication process, the wafer is returned to the stepper for exposure of the wafer dice to another mask.
In operation, at each step, or "cell," the stepper performs a focusing operation, typically by moving the wafer in the z-direction to match the wafer surface with the optimum image plane of the optical system. To perform the focusing operation, certain focus data, specifically, the position of the wafer surface in the z-direction, is measured and the position of the wafer in the z-direction is servo-controlled to nullify the detected focus shift amount. This method has proven to be fairly accurate assuming that the moveable stage, or chuck, on which the wafer rests and the wafer itself are completely flat; however, this is not always the case. In particular, although significant measures are taken to ensure that the fabrication environment is completely free of particles, it would not be impossible for one or more small particles to become trapped between the chuck and the wafer, such that when the wafer is sucked down onto the chuck, the wafer will deform to the particle locally. As a result, the point at which the wafer is deformed will be out of focus, i.e., over-or under-exposed. This effect is typically referred to as a "hotspot," a "chuck spot" or a "chuck ring." Such hotspots later result in dice with substandard geometries, i.e., geometries that are too large, too small, or not resolved. A single hotspot may affect more than one die, all of which must be scrapped or the entire wafer reworked to correct the error.
In many cases, the particle that caused the defect is stuck to the chuck itself, such that more than one wafer will be affected. In view of the fact that the problem may not be detected until visual inspection of the wafers, often several hours later, it is not unlikely that hundreds of wafers could be detrimentally affected before the problem is corrected, thereby significantly multiplying the damage. As previously indicated, the affected wafers must be reworked, resulting in both a productivity and a yield hit, or the affected dice must be scrapped. Moreover, many fabrication facilities are running at full capacity, meaning that every IC chip produced can be sold, and the only way to increase production is to decrease the number of dice that are wasted.
At the present time, the primary method of detecting hotspots is by conducting a post-photolithographic visual inspection of each wafer using a high-power microscope; however, as previously mentioned, if the particle that caused the hotspot is stuck to the chuck, by the time the wafers are inspected, several wafers may already have been affected. One solution to this problem would be to visually inspect each wafer immediately after it comes off the stepper and completes the photoresist development process step. Clearly, this "solution" would result in an unacceptable increase in the time and expense required to produce each wafer, and hence, each die. Moreover, a minor error in focus caused by a hotspot may not be detected by a visual inspection, but could still affect the performance of the resulting IC chips affected thereby.
Therefore, what is needed is a system for enabling the real-time detection of hotspots, which would allow steps to be taken immediately to correct the problem.